Machining and Monitoring Strategies ‘Inque brevi spatio mutander saecla animantum Et quasi cursores vitai lampada tradunt.’ TRANSLATION: ‘The generations of living things pass in a short time and like runners hand on the torch of life.’ Titus LUCRETIUS Carus (94 – 55 BC) [In: On the Nature of the Universe, II] 432 Chapter 9.1 High Speed Machining (HSM) Background to HSM Possibly the first work of note on HSM was that of Salomon who ran a series of applied experiments from the period 1924 to 1931, when a German Patent was granted for this work The Patent was founded upon a series of cutting speed curves plotted against machining temperatures for a range of materials (Fig 214) In these tests Salomon achieved peripheral cutter speeds of 16,500 m min–1, utilising either fly-cutters – for chip morphology data, or helical milling cutters – notably when cutting aluminium Salomon contended that the cutting temperature ‘peaked’ at a specific cutting speed, which he termed the ‘Supercritical speed’, further, when the speed was increased still faster the temperature he noted, decreased Moreover, either side of this ‘supercritical speed’ zone, it was suggested that ‘unworkable regimes’ occurred where the HSS cutters could not withstand the severe forces and temperatures generated As mentioned, when the cutting speed increased beyond this ‘zone’ the temperature reduced to those expected by ‘normal data’ cutting conditions, permitting practical cutting operations to be carried out The problem being with this early HSM work is that any theoretical rationale is not available and the experimental procedures are somewhat unclear, but Salomon can be considered the founder of ultra-high speed machining (UHSM) – taking cutting  ‘Supercritical speed’ mentioned by Salomon when HSM, has never been truly substantiated The practical data was based upon chip morphology* experiments with ‘fly-cut‘ climb-milling of: non-ferrous alloys; ‘soft’ aluminium; red cast brass; the latter being ‘un-machinable’ with HSS cutters between 60 to 330 m min–1 (i.e see Fig 214) As the machining parameters and experimental details only exist as a partial fragment of the original German work This fragmented machining information, is because during the Second World War, unfortunately, the vital details were lost, moreover, none of the participants in this work also surviving to explain how this applied research data was collated *Chip morphology was achieved by situating a heavily waxcoated board, this being strategically positioned to allow the fly-cut chips to stick to the board – during the peripheral climb-milling experimental operation, ready for future analyses data beyond that considered in the so-called ‘Taylorequations’ Effectively it can be established that there were four distinct periods of advancement in the field of UHSM, with the first one being from the early 1920’s to the late 1950’s, with each period during this time and there­ after, being separated by a significant event Obviously during the first period, the work instigated by Salomon (i.e in the 1920’s – alluded to above), was followed by the originally-commissioned United States Air Force (USAF) major research contract, being from 1958 to 1961 Previous to this contract award, little in the way of UHSM work in the preceding decades had occurred, apart from that in the States by Vaughn (1958) Vaughn, shortly became aware of the Salomon Patent, acquiring limited information on this work through the United States Consul in Berlin Vaughn’s (Lockheed) group were also familiar with the many technical references concerning the ‘art’ of oil well tube perforation utilising perforated cutters, these ‘cutting actions’ being employed to perforate oil well casings at explosive speeds Such background work, now meant that Vaughn had ‘set the scene’ for the second period of UHSM development The USAF Materials Laboratory commissioned study – mentioned above, which was awarded to Lockheed (i.e Vaughn’s group), with the objective of evaluating the ‘machining response’ for a selected range of high-strength materials to that of (surface) cutting speeds of up to 152,400 m min–1 The ‘principal aim’ of this ‘USAF-commissioned work’ was to increase producibility, while improving both the quality and efficiency of the fabrication of aircraft/missile com­ ponents Vaughn’s experimental apparatus included the use of modern-day ex-military cannons, which were positioned on ‘railed-sleds’ to minimise subsequent recoil upon firing, while obtaining the desired ballistic exit velocity of the ‘material-slug’ for the cutting speeds Unfortunately, these ‘machining results’  ‘Ballistic cutting speeds’ , were obtained as the cannon fired the projectile (i.e a specific material-slug) at ultra-high speed out of the cannon At the projectile’s exit from the cannon’s barrel, a very robust cutting tool arrangement was situated and held in an accurate position to take a linear cut along the exiting slug’s periphery The evidence of this ballistic cutting action was then recorded by very high-speed photographic equipment – this being both electronically-timed and strategically positioned, for visual dynamic recording and future reference and analysis Machining and Monitoring Strategies 433 Figure 214.  Graphical relationship of high speed machining of metals – according to Salomon’s machining trials [Source: Salomon, circa 1920’s–30’s] 434 Chapter did not indicate how such ballistic speeds might be incorporated into a production application, also, an analytical model of this high-rate cutting phenomenon was not developed In the 1960’s and early 1970’s some consolidation of important machining data occurred, with notable work on UHSM cutting mechanisms, etc., that occurred in various countries being led principally by the USA in work from: Coldwell and Quackenbush (1962), plus Recht (1964); Okushima et al (1966) and Tanaka et al (1967) in Japan; Fenton and Oxley (1967) in the United Kingdom; and Arndt (1972) in Australia However, although there was a general increase global research activity during this period, it was somewhat disparate and mainly of a fundamental, rather than applied nature The third period of HSM development was instigated in the mid-to-late 1970’s by the United States Navy, in conjunction with the Lockheed Missiles and Space Company, Inc., who contracted a series of machinability studies on marine propellers Here, the Lockheed group headed-up by King, were mainly concerned with the feasibility of utilising HSM in a production environment, primarily for aerospace aluminium alloys, then later, working on nickel-aluminium-bronze King’s team at Lockheed, demonstrated that it was economically feasible to introduce ‘highspeed-machining’ procedures into the production environment, thereby realising the significant improvements in productivity with this HSM application Such applied research work, promoted significant activity and interest in this HSM field and, it soon became clear that attention needed to be focussed for all of the subsequent small and diverse research programmes Finally, in these formative years of experimental work into HSM, the fourth period of development began in 1979, when the USAF awarded a contract to the General Electric Company, in this instance, to provide a scientific basis for faster metal removal via HSM and Laser-assisted machining A further contract by the USAF in 1980, was also awarded to ‘General Electric’– the group also being headed by Flom (1980) With this new HSM contract, also being granted to Flom’s group, with the objective to evaluate the production implications of the previous contract Both of these contracts being supported by a consortium of industrial companies and selected universities in the USA – initiating the fourth HSM period of development At around this time (i.e in the late 1970’s), the introduction of computer numerical control (CNC) systems occurred and as a result, they were immediately being fitted to a new range of machine tools, significantly enhancing both their usability and programming capabilities, acting as a catalyst in the development of HSM strategies As these CNC controllers became more sophisticated and processing speeds substantially increased, this meant that the potential for HSM could now be fully realised In recent years, HSM machine tools are just about everywhere in machine shops around the world, where ever there is a need for highly productive part production with very fast cycle times Obviously, on HSM machines as the spindle speeds have increased in association with both tool and their respective workpiece materials (i.e see Fig 215), this has meant that there are now considerable design implications on these machine tools, these topics will now be succinctly discussed 9.1.1 HSM Machine Tool Design Considerations  ‘Marine propeller manufacture’ , whether from a wroughtsolid material, or finishing-off a casting, is probably the most difficult and complex multi-axes milling operation that can be undertaken – due to the fact that the propeller surfaces to be machined are continuously changing their geometry as they are essentially parabolically-curved Invariably, the part geom­etry is typified by say, the NACA/NASA Standard 16-021 ‘aerofoil cross-sectional profile’ , which for high-performance propellers are exacerbated by normally having both considerable rake and skew to each blade – creating an exceedingly complex geometry and fillet where the boss and blades intersect* * See: Smith and Booth (1993) paper – in the references, which goes some way to explain the propeller manufacturing subject, and for more detailed multi-axes machining and for subsequent machined propeller measurement information Prior to a discussion concerning machine tool design factors that must be addressed, before to fully-implementing this HSM technology, one might ask the question: ‘Why we need to rotate cutters at such high spindle speeds?’ There are a number of advantages that can accrue from adopting such a progressive produc-  USAF contract to the General Electric Company in 1979, was: F 33615-79-C-5119 – for a feasibility study into the fast metal removal operations by HSM and Laser-assisted machining Machining and Monitoring Strategies 435 Figure 215.  The chronological development of cutting tool material introductions, which had an influence on high-speed cutting trends [Courtesy of Yamazaki Mazak Corporation] tion machining strategy (i.e see Fig 216) and, they can be succinctly summarised as follows: • Direct benefits – improved machining efficiency, – reduction in cutting tool varieties, – reduction in distortion of workpiece, – effectiveness of swarf removal • Indirect benefits – quality of finish improved, – increased cutter life, – reduced changes in material properties, – capability of machining thin walls/sections These production improvements are by no means all that occur, as invariably, due to the superior machined surface texture, the final part surface may not need to be deburred – a significant real saving on complex component geometries Moreover, by employing an HSM strategy, more simple fixturing can be utilised, as the actual tool forces are significantly lessened It is an established fact that with higher rotational cutter speeds the resulting cutting forces and tool push-off are considerably reduced In order to benefit from these improved cutting practices, the machine tool’s axes must have both faster acceleration and deceleration – see Fig 221, more will be mentioned concerning these important dynamic aspects of a machine tool’s operational performance shortly Many of today’s conventional and HSM machine tools, are based upon a modular design concepts (Fig 217a) This modular design philosophy, allows the machine tool builder the opportunity to standardise certain features over a range of machine tools Such practice benefits the manufacturer and consumer alike, by reducing design 436 Chapter Figure 216.  Diagram illustrating the main benefits to be gained from adopting a high-speed machining strategy [Source: Smith, CNC Machining Technology, 1993] and development costs, while minimising the customer’s purchasing costs, yet still allowing more attention to be given to the detailed design of each ‘module’ in the machine So, an identical column, or table may be common to a variety of machines and this trend can often be seen across a whole product range of machine tools Not only are the major castings, or large fabrications manufactured by employing modular concepts, but this design philosophy also allows any other smaller components to be standardised and fitted accordingly Such as: the recirculating ballscrews; servo-motors; linear scales – if fitted; etc.; plus other standardised items to be built into the constructed machine tool ranges In order to minimise ‘stick-slip’  in the slideway motions on heavy moving cast and fabricated members –  ‘Stick-slip’ or ‘Stiction’ , is the jerky-motion between sliding members due to the formation and destruction of junctions [due to localised pressure-effects] (Kalpakjian, 1984) Machining and Monitoring Strategies 437 Table 14.  Comparison of different drive systems for machine tools CONTRIBUTIONS: Leadscrew: Ballscrew: Belt-drives: Linear motor: Noise Quiet Noisy Quiet Moderate Back-driving Self-locking Backlash Increases with wear Constant Increases with belt wear Negligible Repeatability ±0.005 mm ±0.005 mm ±0.004 mm Best (2,000 N µm–1, so if greater kinematic and dynamic performance is required, then it might be necessary to utilise linear-motor drives A tabulated table for suitable comparisons of the various types of motional drive systems available today, is presented in Table 14 It has been widely-reported that either a high-quality lead-, or ball-screw having rotary encoders, will have a unidirectional repeatability to within 6-to- Where: L = overall length of the ‘elastic member’ (Galyer and Shotbolt et al., 1990) Example: If a machine tool’s structural moving member is 1,000 mm in length and it is situated on a much longer ‘bedway’ , then the ‘Tychoways’ should (ideally) be symmericallypositioned (i.e fixed to the moving member) at a distance of 554 mm apart – in order to obtain the minimum of elastic distortion as it moves backward and forward along the hardened bedway 438 Chapter Figure 217.  Typical ‘modular design’ and construction of machining centres, with ‘ballscrews’ and ‘tychoways’ [Courtesy of Cincinnati Machines] Machining and Monitoring Strategies 7 µm However, if linear encoders are fitted this minimises any potential ‘ballscrew wind-up’ , although the problem of an ‘Abbé -error’  is still present Yet another source of machine tool-induced problems are more specifically termed ‘uncertainties’ , while ‘hysteresis’  can also contribute to the overall ‘error budget’ Thus, hysteresis may result when the same position has been commanded by the CNC controller,  ‘Abbés Principle’ – was derived by Professor Ernest Abbé in 1890 (i.e having studied and graduating from the University of Jena) and is still valid today The Abbé principle simply states that: ‘The line of measurement and the measuring plane should be coincident’ An example of this ‘Abbé Law’ wellknown to engineers the world over, shows that a conventional micrometer calliper ‘virtually-obeys’ the ‘Abbé principle (i.e there is a small ‘cosine error’ present – which can usually be ignored), whereas, the Vernier calliper has a much larger offset between the fixed and moving jaws – where the component being measured is situated, to that of the beam – where the scale’s reading for this measurement is taken Ideally, both measurement and reading should be lined-up, without an offset [Sources: Busch, 1989; Whitehouse et al., 2002]  ‘Uncertainty of machine tool positioning’ – the question often asked in calibration-related tasks, is: ‘What is measurement uncertainty?’ Uncertainty of measurement refers to the doubt that exists about any measurement; there occurs a margin of doubt for every measurement This expression of measurement uncertainty raises other questions: ‘How large is the margin?’ and ‘ How bad the doubt?’ Hence, in order to quantify uncertainty two numbers are required: (i) being the width of the margin – its interval, (ii) plusits confidence level NB  This latter value states how sure one is that the actual value occurs within this margin.For example:On a CNC machine tool, the command may be to move the X-axis slideway 1000 mm plus, or minus 0.05 mm at the 95% confidence level This uncertainty could be expressed, as follows: X-axis slideway motion = 1000 mm ± 0.05 mm, at a level of confidence of 95% In reality, what this statement is implying to either the programmer, or calibration engineer is that they are 95% sure that the actual X-axis position now will lie between: 999.95 mm and 1000.05 mm Many factors can contribute to the overall ‘error budget’ as it is sometimes known, but this is beyond the scope of the present discussion – see references for further information (Smith, 2002)  ‘Hysteresis’ , can be defined as: ‘The difference in the indicated value for any particular input when that input is approached in an increasing input direction, versus when approached in a decreasing input direction.’ (Figliola and Beasley, et al 2000)For example:Hysteresis usually arises because of strain energy stored in the system [i.e in this case, within the machine tool], by slack bearings, gears, ballscrews, etc (Collett and Hope, 1979) 439 but from opposite directions, causing the motion system to creep by an amount larger than the backlash alone (i.e the hysteresis) This effect is the result of unseen working clearances and compounded by the machine’s elastic deformations, although by pre-loading the ballscrews it will minimise some of these effects In HSM machining applications, all ballscrew and indeed any screw-driven systems have some additional limitations, such as its ‘critical speed’ of rotation At the critical speed, a ballscrew starts to resonate10 at its first natural frequency (i.e termed ‘whipping’) Hence, the critical speed is proportional to the ballscrew’s diameter and is inversely proportional to the distance between the screw’s supports – squared For a very long and slender screw-driven machine tool application with wide supports, here, most recirculating ballscrews would have a critical speed of approximately 2,500 rev min–1 It should also be said, that for many ballscrews assemblies they can be rotated at higher rotational speeds than the 2,500 rev min–1 previously mentioned, before they reach their critical speed, but for very fast accelerations and decelerations, then they become increasing challenged In fact, on some HSM machine tool configurations, multi-start ballscrews have been employed to increase linear response, but here the ‘critical speed’ will probably be lessened – due to reduced inherent ballscrew stiffness Even ‘matched’ twin ballscrews have been fitted to HSM machine tools – to minimise any potential ‘yawing motions’ at high linear speed by the moving member along the machine’s bedway These ballscrew limitations are probably why linear-motional drives are becoming a realistic alternative, but they are only fitted at present, to high capital cost HSM machine tools One of the bi-products of HSM’s greater stock removal rates, is the excess volume of hot swarf which must be speedily and efficiently removed from the vicinity of the machine – which is more readily achieved for horizontal machine tool configurations Thermal effects in general on any machine tool become a problem, particularly as many milling spindles utilise direct-drives, with the motor being mounted in-situ with the spindle Here, the spindle motor creates heat, the thermal effects of which can be analysed by either 10 ‘Resonant frequency’ , can be defined as: ‘The frequency at which the magnitude ratio reaches a maximum value greater than unity.’ (Figliola and Beasley et al., 2000) 440 Chapter a three-, or five-probe spindle analyser, as depicted in Fig 218 along with a typical case-study for a spindle’s thermal drift graph Not only can such spindle analysis equipment determine a spindle’s thermal growth, it can also detect: thermal distortion; spindle error; machine resonance; vibration; plus repeatability In the casestudy graphically-depicted in Fig 218b, the variation in the machine tool’s temperature is the major cause of positioning error This thermal drift graph offers-up a number of significant questions that need to be addressed, such as: ‘How long does it take for the machine tool to stabilise?’; ‘How much Z-axis growth does the machine produce at full spindle speed?’; ‘How far has the machine’s displacement become, because of distortion in the machine tool structure?’ Spindle analysers are able to operate at rotational speeds varying from to 120,000 rev min–1, making them an ideal tool for condition monitoring diagnostics on machine tool’s equipped with HSM spindles for subsequent analyses In Appendix 15, a trouble-shooting guide has been produced to high-light: problems; causes; tests; etc.; that can be obtained from analyses by employing machine tool spindle analysers In Fig 219, the polar plots illustrated show how the spindle analyser has the ability to evaluate both a new and rebuilt machine tool spindle’s condition In the spindle error indicated on the polar plot depicted in Fig 219a, it illustrates here, that a very badly rebuilt spindle is simply not acceptable in this state This poor spindle performance, is in the main, largely the result of significant radial variability (i.e total error: 12 µm @ 4,005 rev min–1), which would severely compromise any cutting tool’s machining performance Conversely, in Fig 219b, a well-worn spindle assembly is shown prior to rebuild, having a total error of 4.6 µm @ 1,702 rev min–1, after its ‘correct’ rebuilding (Fig 219c), the total error has been drastically reduced to a total error of 1.9 µm @ 1,700 rev min–1 Visually, the differences between these two polar plots is quite astounding, in that both the asynchronous and average errors present have virtually disappeared in the latter case, making it ‘as-new’ and, ready to perform significant machining service It is well-known fact by machinists familiar with their older and heavily-utilised machine tools, that certain machine spindles will run smoother and produce better machined roundness on workpieces if they are run at the so-called ‘sweet-spot’ , equally, the quality of the parts produced will be affected if the spindle is run at its ‘sour-spot’ By utilising a spindle analyser, significant information can be gleaned from such rotational analyses, allowing speed ranges to be selected which would currently optimise the present status of the spindle’s condition, prior to its potential rebuild – when called for at a due date in the future For most machine tools today that are involved in HSM activities, in general the spindle cartridges are of three distinct design configurations – which incidentally not normally include ball bearing spindle types11, such as: • Magnetic ‘active’ spindles (Fig 220a) – typically might have a cartridge with a speed range from: 4,000 to 40,000 rev min–1, delivering 40 kW continuous power, peaking at over 50 kW Such ‘active’ magnetic spindles can maintain 1 µm maximum run-out, by digital control of the series of specifically-orientated magnetic currents – being initiated by radial and axial sensors, that continuously monitor the spindle’s rotational axis position 10,000 times second–1, NB  Further refinements to the spindle occur, with these temperature-controlled milling spindle cartridges maintaining dynamic balance, regardless of the milling cutting loads imposed, this latter factor being an important criterion when attempting to reduce cutting tool vibrational effects However, these ‘active’ spindles are not cheap to purchase and run, with another negative effect being that they are normally rated for only several thousand hours operational running time Such cartridges come with a variety of rotational speed ranges and spindle power outputs • Pneumatic spindles – have been available for many years, with aerostatic bearings equalising the forces exerted while cutting and remaining centralised within the spindle’s housing, yet still achieving dynamic rotational balance, 11 ‘Ball bearing spindle designs’ , are not normally specified for HSM operations, because at around 20,000 rev min–1 – this being ‘mechanically-set’ as the upper rotational velocity of such spindles So, due to these high rotational speed effects and of the combination of centrifugal forces, it means that at approximately 80 m sec-1 rotational speed, the balls will lose contact with the journal walls As a result of this loss-of-contact the hardened balls and raceways will rapidly wear out (i.e the results of so-called: ‘Brinelling’ in the raceways, creating both poor circumferential wear patterns, delaminations and associated frictional effects – leading to major debilitating spindle roundness modifications.) (Smith et al., 1992) Machining and Monitoring Strategies Figure 218.  Machine tool spindle analysis system [Courtesy of Lion Precision] 441 ... electronically-timed and strategically positioned, for visual dynamic recording and future reference and analysis Machining and Monitoring Strategies 433 Figure 214.  Graphical relationship of high speed machining. .. fast metal removal operations by HSM and Laser-assisted machining Machining and Monitoring Strategies 435 Figure 215.  The chronological development of cutting tool material introductions,... model of this high-rate cutting phenomenon was not developed In the 1960’s and early 1970’s some consolidation of important machining data occurred, with notable work on UHSM cutting mechanisms,